Operation In Our Sites, led by U.S. Immigration and Customs Enforcement (ICE), was launched in 2010 to crackdown on websites offering pirated content or selling counterfeit goods. Under the program, the government has seized at least 839 websites, including major sites like Megaupload and NinjaVideo, which offered popular movies and TV shows for free.

Visitors to the seized websites now see only an imposing government banner with an anti-piracy warning and the seals of various federal agencies.

The seizure banners have been viewed more than 103 million times since the program launched, according to ICE.

Entertainment trade groups, such as the Motion Picture Association of America (MPAA) and the Recording Industry Association of America (RIAA), have applauded Operation In Our Sites for fighting back against online copyright infringement, which they argue is stealing profits from artists and destroying jobs.

But the lawmakers expressed concern that government officials are being overly aggressive and are seizing legitimate websites without giving the owners a chance to defend themselves.

In 2010, ICE seized hip-hop site Dajaz1, but evidence later showed that much of its content was legal. Authorities returned the site, but only after holding it for more than a year.

“As a result of ICE's improper targeting and RIAA's failure to respond to government requests for assistance, the censorship of what appears to be a legitimate website was unnecessarily prolonged while the website owner was unable to get his day in court to present his defenses,” the lawmakers wrote.

Authorities also droppedcharges against Spanish sports site Rojadirecta this week and returned its domain.

The lawmakers pressed Holder and Napolitano for details about how the government determines which sites to seize and whether agents take any steps to ensure that the sites are actually illegal. They also asked whether the administration has changed any of its policies after the Dajaz1 case.

Sherwin Siy, vice president of legal affairs for consumer advocacy group Public Knowledge, argued that the government should have to get a judge to approve an injunction to take down a site. Under the current system, the authorities confiscate the websites as asset forfeiture, much like police might seize a drug dealer's car after arresting him.

“It's always better when the defendant can show up and defend themselves,” Siy said.

He said he hopes the lawmakers' letter will spur hearings and possibly lead to legislation that would add more legal safeguards for websites accused of copyright infringement.